P-19 TECH TALK:
FIXED SHEETLEADS, GENOA TRACKS AND THE CDI REEFING FURLER
Sheet: A line (rope) used to trim a sail.
Sheetlead: A fairlead or block (pulley) used for running jib or genoa sheets. May be fixed in place or slide on a track.
Genoa Tracks: Tracks with sliding cars and sheetleads used for running the jib or genoa sheets.
Sheeting angle: The angle defined by the centerline and a line drawn from the headstay to the sheetlead.
Clew: The aft lower corner of a sail
Tack: The forward lower corner of a sail.
Foot: The bottom edge or section of a sail
Leech: The aft edge or section of a sail
Pendant: A short piece of wire or rope used to extend a halyard or sail. Can be used between the tack of a jib and the chain plate to set the jib up from the deck, thereby raising the clew. AKA pennant.
Coaming A low wall around a cockpit.
Cabin bulkhead: As used in this article, the aft cabin wall located at the front of the cockpit.
P19 Standard Jib: A jib of approx. 32 S.F. with a high clew. It does not overlap the mast. AKA storm jib.
P19 Lapper: A jib of approx. 65 S.F. with a high clew. It overlaps the mast.
P19 Genoa: A foresail of approx. 110 S.F. with a low clew. Its long, low foot extends aft to the cockpit.
WHY USE TRACKS AND CARS?
Why would you want tracks, especially with a CDI unit installed? With tracks and adjustable cars, you can fine-tune the shape of the headsail for efficiency and you can fine-tune the shape of the "slot," the space between the headsail and the main.
The position of the sheetlead determines how effectively the jib sheet pulls down and aft on the clew of the jib. If the lead is too far forward, the leech of the jib is too tight and the foot is too loose. This stalls the bottom of the jib, chokes the airflow through the slot and causes turbulence over the mainsail. When you're pointing, this may backwind the mainsail. If the lead is too far aft, the leech is too loose and the foot is too flat. This stalls the top of the jib and spills wind.
Ideally, on a sloop rig, you want the curve in the leech of the jib to match the shape of the draft of the mainsail. On a fractional rig the mainsail, not the jib, produces most of the power. With a badly shaped slot, the Potter's mainsail can't perform up to its full potential and the boat will feel under-canvassed. (see foot note 1)
When you reef the headsail on a CDI unit, the jib sheet pulls the foot of the headsail too tight, and the leech gets too loose. It's the same as having the sheetlead too far aft. The leech gets too loose and the airflow over the top of the jib is turbulent. Therefore, when you reef the jib, you usually need to move the sheetlead forward.
(This is a very simplified explanation. You can read more in Maloney's Chapman Piloting, 62nd edition, page 228-235; and Rousmaniere's Annapolis Book of Seamanship, 2nd edition, starting on page 70. Illustration by Mark Smith, from The Annapolis Book of Seamanship)
Left: The lead is too far aft. The top of the jib stalls easily (the top telltale lifts first when you sheet in). The leech is too loose and the foot is too tight.
Middle: The lead is too far forward. The bottom of the jib stalls easily (the bottom telltale lifts first when you sheet in). The leech is too tight and the foot is too loose.
Right: When all the telltales all lift at the same time as you sheet in, the lead is in the right place.
FIXED SHEETLEADS WITHOUT THE CDI
On the newer factory-standard P-19's, the sheetlead for the both the lapper and the standard jib is a single stand-up block on the side deck about 4" aft of the shrouds, close to the cabin wall. For the genoa, you need to order the optional tracks or install a sheetlead yourself. The factory installs the optional genoa tracks on the outward-facing vertical wall of the cockpit coaming, from the front of the cockpit and running aft for about a foot. The optional tracks come with a fixed stand-up block on the side deck of the cockpit, which provides a better lead to the camcleat or optional genoa winches on the side deck of the cockpit.
For fixed sheetleads, Jerry Barrilleaux's HMS-18 "Sunshine" is a good boat to copy. Jerry's a renowned Potter skipper and the Commodore of the Potter Yachters. His big Potter is tuned and rigged almost perfectly. Jerry says running the jib sheets inside the shrouds for the standard jib and lapper is very important for pointing higher. For the standard jib, he has a stand-up block about 1 or 2 inches away from the cabin wall, about 4 or 5 inches aft of the shrouds. He uses a 6-inch pendant made of wire at the tack of the standard jib in order to get the correct clew height for his sheetlead position. For the lapper and genoa sheetleads, Jerry has an 18-inch track on the side deck, running aft from the back of the rear cabin window. According to Jerry, this location works better than the factory's for the lapper. He attaches the tack of the lapper directly to the chain plate. He rarely moves the block for sailing with the lapper, so it might as well be fixed about 6 inches aft of the corner of the rear window.
FIXED SHEETLEADS WITH THE CDI
Most P19's with the standard jib or lapper on the CDI roller/reefer don't have tracks. They come from the factory with a stand-up block installed just aft of the shrouds for the lapper and standard jib. For the genoa, the factory recommends the optional track on the cockpit gunwale. Their owners report that they perform very well and they are a pleasure to sail.
If you're retro-fitting a CDI or want to experiment with moving the fixed sheetlead on your boat, try this. Start by unfurling the sail on land and pull the jibsheet back to the deck so that it's perpendicular to the headstay. Mark this spot on the deck. When the sail is rolled up, the clew moves along this line. With the sheet lead located at this spot, the horizontal angle of the jib sheet doesn't change much as you role the sail. Also, mark the locations of the sheetleads that Jerry uses on his boat.
Next, go out sailing in moderate conditions and experiment by having a friend hold the jibsheet near the marked spots while you sail. Try it with the sail fully extended as well as partially rolled up. The cut of your sail and the conditions you sail under will affect your test results, so test several inches fore and aft of the marks on your deck. By experimenting with different locations, you can find the best place for mounting a fixed sheet lead for each headsail you use on your CDI.
ADJUSTABLE TRACKS WITH THE CDI
Where should you install tracks? For the lapper and the standard jib, there are at least two good locations. Most books on sailplans say the ideal placement of the sheetlead for a standard jib (non-lapper, less than 100% of the foretriangle) is about 8-10 degrees off centerline and around 10-12 degrees for a lapper.
No matter where you put the tracks on your P-19, you should run the jib sheets for the lapper and standard jib inside the shrouds to improve upwind performance. (Actually, I run the sheets for the lapper between the two shrouds! )If you run the jib sheets outside the shrouds, the sheeting angle will be about 30 degrees off the centerline, which is much too wide for most sailing conditions. For the genoa, you have to run the jib sheets outside the shrouds because the foot of the sail is so long that there's no other option.
If you want one set of tracks for use with all 3 jibs (the tiny storm jib, the lapper, and the 150% genoa), you should put tracks on the side deck, up against the cabin wall, from approximately the middle of the front cabin window to about 18" aft of the cabin bulkhead. It's about 16-18 degrees off centerline for the standard jib and lapper. The ergonomics of placing them here are really excellent, especially if you have winches on the coaming. This is what I'd recommend for most people who use all three sails.
If you want to point higher with the lapper and the small jib, the best place for tracks is on the cabin top, outboard of the handrails. For the lapper, run the sheets either inside both shrouds or between the two side shrouds. For the small jib, run the jibsheets inside both shrouds. That's a reasonable sheeting angle of 12-14 degrees off centerline. To get an even tighter sheeting angle, you can tighten the windward (lazy) jibsheet just a bit. It acts like a barberhaul, pulling the clew towards the centerline a few degrees.
One advantage to the cabintop tracks is that you can reach the cars very easily from the cockpit to adjust the position of the car, they're not underfoot, the cabin top is very stiff and strong there, and the cars slide easily. That's the location my rigger and sailmaker both recommended. The only drawback is that most P19's don't have any winches on the cabintop to help you trim the sheets in heavy winds (I installed winches on the cabintop of my P19).
A cabin top track is great for the standard jib and lapper with their high clews, but obviously it's too far forward for the genoa's long foot and too high for its deck-sweeping clew. If you only install one set of tracks on the cabin top, you'll need to install a single fixed block or a short piece of track on the side deck near the cabin bulkhead.
Another location for tracks is on the vertical wall of the cabin and cockpit coaming, starting at the front of the rear window and extending a foot past the cabin bulkhead. That's about 15 degrees off centerline and the track won't be underfoot. So far, so good. However, you won't be able to reach the front of the track from the cockpit and you will probably need another stand-up block somewhere to get a good lead to the cleat. And with the high clews of the standard jib and lapper, the car might not slide easily under load or the block could bend the padeye of the car. You'd have to solve these problems if you choose to install the tracks here.
Before installing tracks on my P19, Redwing, I experimented while sailing on San Francisco Bay with the owner of UK Sailmakers of Alameda. We held or tied the jib sheets in all possible locations. We tested every possible way to run the sheets. We used the lapper, both full-sized and partially reefed on the CDI. Redwing pointed highest and sailed fastest with the jib sheets run inside the shrouds to the cabin top. Off the wind, the jib sheets rubbed against the shrouds, but that didn't affect downwind performance adversely.
In summary: If you want top-notch pointing performance from your P19, particularly with the CDI reefing furler, you should consider cabin top tracks for the standard jib and lapper, with a separate fixed block or track for the genoa further aft on the cockpit gunwale. For all around performance with all three sails (small jib, lapper and genoa) your best option is one long track on the side deck, from the front cabin window to 18" past the cabin bulkhead.
CHOICES, CHOICES, CHOICES
The Potter 19 is a great sailboat. With the basic rigging (hanked-on jib and fixed sheetleads), you have a very affordable mini-cruiser that handles beautifully. Or you can install optional hardware like a CDI furler and custom genoa tracks and turn your P-19 into a high-performance mini-cruiser. The choice is entirely yours. Either way, you'll love sailing your Potter.
Footnote 1: The old "Slot Theory" is no longer believed to be valid. The famous book by C.A. Marchaj, Aero-Hydrodynamics of Sailing, page 638, says that, "Many problems concerning the interference between a mainsail and a jib were clarified by A. Gentry who explained correctly, for the first time, the jib-mainsail interaction effect." Gentry's scientific articles on aerodynamcis, published in the 1970's, forever disproved the old "Slot Theory".
For an introduction to and correct understanding of modern sail theory, you might want to read Chapter 5 of the book, The Art and Science of Sails, by Tom Whidden and Michael Levitt, which is based on the work of Gentry. Sailors who are interested in greater detail may want to read the published papers of Arvel Gentry, who published the pioneering work on sailing and aerodynamics in the 1970's on which modern sail theory is based. Gentry also wrote a series of article for Sail magazine in 1973, which present much of the modern theory. These articles have been republished in the book, The Best of Sail Trim.
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